This invention relates to a solid state video camera which uses a semiconductor element, for example, a charge coupled device, as an image sensor, and more particularly, is directed to a solid state video camera, as aforesaid, which has an automatic processing circuit to generate a high quality image.
Solid state video cameras which use charge coupled devices (hereafter CCDs) as image sensors are well-known. The most recently developed cameras, due to their capability of operating at low light levels and providing a high resolution, have been applied to a wide range of industrial applications, including facsimile transmission, automatic optical character recognition, bar-code reading and robotics. Other applications presently being contemplated include industrial measurement and inspection of such products as hot steel, sheet surfaces, and even processing (i.e. sorting) vegetables.
A CCD image sensor, as is known in the prior art, typically includes a silicon substrate covered by an insulating layer such as silicon dioxide and having thereon an array of closely-spaced gate electrodes formed from, for example, doped polysilicon material. The gate electrodes are interconnected in rows or columns, which are electrically connected to both a CCD address circuit for sequentially applying gate electrode voltages, and to a data readout register.
When a CCD is used as an imaging device, light reflected from the object to be imaged, e.g. a document, is projected on the CCD. This luminous energy generates a corresponding pattern and number of charge carriers in the bare areas of silicon (invariably called photosites, picture elements or pixels) as explained below. During the so-called integration time, which is analogous to the exposure time in conventional cameras, the CCD address circuit applies voltages to the gate electrode array to provide a pattern of potential wells which attract charged minority carriers. A charge pattern will form in the potential wells under the various photosites nearest to where the charges are produced. The number of charge carriers which accumulate in each potential well during the integration time is proportional to the amount of light reaching that well and this, in turn, is proportional to the light intensity and the duration of the integration time. Thus, a spatial pattern of charge carriers corresponding to an optical image is produced by the CCD.
The above CCD image sensor uses an area sensor consisting of a matrix of photosensors addressed in two-dimensional coordinates by decoders or shift registers so as to select each individual element in turn and read out its charge. Area sensor CCDs having as many as 380 columns of 488 photosites are now available. However, because of various limitations, this approach has not been popularly accepted.
Another approach is to use a CCD imager that senses a single line. This linear sensor consists of a single row of sensors and it may be used in a two-dimensional sense by using a mechanical scanner such as a rotating mirror or moving the object to be scanned in front of the sensor.
Regardless of whether the CCD image sensor is a matrix arrangement or a linear array, the image is typically read out from the sensor as one or more separate fields or channels. For example, in a dual channel linear array CCD image sensor all the odd-numbered photosensor elements are connected together constituting one channel and all the even-numbered photosensor elements are connected together constituting the second channel. Each of these channels in the dual channel CCD image sensor contains an internal black reference and white reference and only one-half of the projected graphical image information thus requiring appropriate interlacing of the two channels to obtain the full image. For a detailed description of CCD interlacing techniques, reference is made to U. S. Pat. No. 3,911,467, issued to Levine et al.
Each channel of a multi-channel CCD image sensor is provided with an output analog amplifier which is adjusted to the required offset and gain values dictated by the CCD manufacturer tolerances, illumination variations, integration time, etc. As a result of these variations, an imbalance between the channels may occur introducing unwarranted distortion and artifacts in the image signals generated by the CCD array resulting in erroneous or unsatisfactory imaging of the object.
One way of solving this problem is by means of gain and offset circuits having manual gain and offset adjustment potentiometers to achieve a balanced condition between the two output levels of the various channels. However, this technique suffers from several disadvantages including drift of potentiometers due to temperature variations and mechanical vibrations and the necessity for time-consuming interactive potentiometer adjustment occasioned by waiting for the human operator to form a judgment as to whether the balanced condition has been achieved. Furthermore, the resultant correction of the video signal is quite gross and imprecise. Such reliance on human intervention is not desirable in document imaging at a high rate of 200 or more documents per minute or in other industrial applications where accuracy is critical.
Another way of solving the above channel imbalance problem is by means of an image signal restoration and gain control system disclosed in U.S. Pat. No. 4,216,503, issued to Wiggins. In this system, the black and white reference signals are isolated and processed by a microprocessor in accordance with a pre-established table to obtain an offset subtraction factor and a gain multiplicand. Using these factors, offset and gain are, respectively, corrected by means of a subtractor and a multiplier.
The Wiggins system, however, suffers from several disadvantages. It requires a microprocessor and its associated circuitry which is rather complex and expensive. Second, the microprocessor must perform averaging of blocks of 16 or more white and dark signals to determine the subtraction and multiplication factors which is rather time-consuming and not suitable for high-speed and accurate image processing. Third, the microprocessor needs special software to accomplish the required functions which may have to be rewritten whenever the CCD is changed. Another disadvantage with the Wiggins system is that the subtractor and multiplier are not incorporated in a feedback loop and, accordingly, offset problems in these devices due to temperature drift, etc. are not corrected. Yet another disadvantage is that the subtractor and multiplier are quite expensive, thus rendering the overall cost of the circuit expensive.